1. Field of the Invention
The present invention relates to a synthetic silica glass manufacturing apparatus for manufacturing silica glass, and more particularly, to a synthetic silica glass manufacturing apparatus for manufacturing silica glass optical members, such as lenses and mirrors, for use in the wavelength range of 400 nm or less or in the range of 300 nm or less in photolithography technology.
2. Discussion of the Related Art
Currently, an exposure apparatus, called a stepper, has been used for photolithography technology to expose and transcribe very fine patterns of integrated circuits onto a wafer made of silicon. Due to recent trends towards greater integration of LSI, shorter wavelengths, from the g-line (436 nm) and the i-line (365 nm) to excimer lasers, such as KrF (248 nm) and ArF (193 nm), have increasingly being used as a light source for the stepper.
LSI has developed into VLSI (Very Large Scale Integration), which is a subset of LSI. For example, in the case of DRAM as an example of VLSI, the capacity has been increased as 1K.fwdarw.256K.fwdarw.1M.fwdarw.4M.fwdarw.16M.fwdarw.64M.fwdarw.256M.fwdar w.1G. Due to this increase in capacity, the desired resolution of the steppers in exposing fine lines has been significantly increased as 10 .mu.m .fwdarw.2 .mu.m.fwdarw.1 .mu.m.fwdarw.0.8 .mu.m.fwdarw.0.5 .mu.m.fwdarw.0.35 .mu.m.fwdarw.0.25 .mu.m.fwdarw.0.18 .mu.m.
Accordingly, high resolution and deep focal depth are required for the projection lenses for steppers. In general, for the illumination optical system and the projection optical system of steppers, multi-element optical glass having a high transmittance is used as the lens material for the i-line. In the case of excimer lasers, such as KrF and ArF lasers, synthetic silica glass and single crystal fluoride, such as CaF.sub.2 (fluorite), are used instead of conventional optical glass.
In particular, in mass-production lines for microprocessors which are used as a VRAM having a high capacity of 16M or more, steppers using an excimer laser that is capable of exposing a line width of 0.25 .mu.m have been introduced. For the optical devices of such ultraviolet light lithography (lens materials used in illumination or projection optical systems), a high purity synthetic silica glass is used in order to achieve a high transmittance within the ultraviolet light range.
One of the known effective manufacturing methods of this synthetic silica glass is a flame hydrolysis method. In the flame hydrolysis method, a silicon compound (the material of synthetic silica glass) is supplied to an oxy-hydrogen flame formed by a combustion burner, and hydrolyzed to produce minute silica particles. The minute silica particles are then deposited, fused, and are vitrified.
A synthetic silica glass manufacturing apparatus that performs this synthesis method has a structure similar to that of so-called "Bernoulli furnace." The apparatus has an outer wall having a double-wall structure to efficiently store heat and has an exhaust. The synthesis is conducted by maintaining a high temperature of 1000.degree. C. or more inside the furnace. This manufacturing apparatus is structured by assembling fireproof members (fireproof parts) inside the furnace (furnace frame) and has a target located for forming an ingot thereon and a burner pointing towards the target for synthesizing silica glass.
The target is arranged on a stage in such a way as to be vertically movable. By lowering the stage at the same speed as the growing speed of the silica glass, an appropriate distance between the burner and the synthesis surface of the silica glass can be maintained. In addition, in order to supply a flame temperature (supplied thermal energy) from the burner uniformly to the synthesis surface of the silica glass during the silica glass synthesis, the target is rotated and swayed at the same time.
However, the silica glass synthesized using the synthetic silica glass manufacturing apparatus described above often has a non-uniform refractive index profile for various reasons. One of the causes of the irregularity is the fluctuation of various conditions (changes in synthesis conditions) during the synthesis of the silica glass. Among others, a positional deviation of the target relative to the furnace and mismatch of the growth speed of the silica glass with the lowering speed of the stage are significant factors.
In general, a synthetic silica glass manufacturing apparatus is constructed of a furnace and an elevation system. The furnace and the elevation system are structured such that they can be separated and placed on the floor separately. The elevation system supports the stage such that the stage is vertically movable, and the target is rotatably mounted on the stage of the elevation system. Therefore, in order to avoid inducing the above-mentioned causes, the elevation system is aligned at the beginning of the synthesis so that each condition of synthesis is fixed to a predetermined state.
There are cases where the elevation system needs to be moved away from the furnace; for example, when the apparatus undergoes maintenance. Therefore, a plurality of saucers is provided for supporting the legs of the elevation system on a floor below the synthetic silica glass manufacturing apparatus. The number of the saucers corresponds to the number of legs. After the maintenance, the position of the elevation system relative to the furnace is aligned by placing the legs of the elevation system on the saucers.
However, the saucers are formed as a disk shape with a planar surface in the conventional art, and therefore, by merely placing the legs on the saucers, it is difficult to reproduce the aligned condition which is previously set at the beginning of prior synthesis (alignment at the beginning of the synthesis). This necessitates additional time for re-aligning when synthesis is re-started.
For the elevation system, considering the load of the ingot after the synthesis is finished, it is desirable to arrange the center of the stage to correspond to the rotating axis of the target. Here, a system like a jack may be applicable to the elevation device. However, the alignment precision of such a system is low and it requires a large space around the lower portion of the stage. Therefore, such a system can neither be arranged directly below the stage, nor can it have a long stroke. Accordingly, such a system cannot be effectively used for the synthetic silica glass manufacturing apparatus.
In addition, in order to vertically move the target (stage) with high precision, a highly precise actuator is necessary. However, the ingot of the synthetic silica glass exceeds as much as 200 kg at the end of the synthesis. Also, the target for supporting the ingot and the rotation and translation system unit weigh approximately 200 kg. Therefore, a suitable elevation system requires a load resistance of 400 kg or more as well as very little tolerable positional deviation during its elevation motion.